Boosting NIR Laser Marking Efficiency of a Transparent Epoxy Using a Layered Double Hydroxide

Efficient near-infrared (NIR) laser marking on transparent polymers like polypropylene, epoxy, and polyethylene has posed a big challenge due to their lack of absorption in the NIR. Currently, inorganic additives are used to improve NIR laser marking efficiency, but they come with issues such as toxicity, high loading requirement, adverse effects on color/opaqueness, and the need for low laser head speeds. Herein, we report a new strategy of incorporating a food-grade, Mg2Al-CO3 LDH as a boosting coadditive alongside the commercial NIR laser marking additive (Iriotech 8815) in an epoxy system. Our findings demonstrate that the incorporation of Mg2Al-CO3 LDH can significantly increase both the darkness and contrast of marking even at high laser head speed (5000 mm/s), while minimizing surface damage. Notably, by replacing 95% of Iriotech 8815 with Mg2Al-CO3 LDH, an epoxy plate can exhibit high transparency, while producing dark, sharply defined markings with excellent readable QR code markings at high laser speeds. This result offers a promising solution for enhancing high-speed NIR laser marking on transparent polymers with additional advantages of lower toxicity and cost and with minimal optical interference from high additive loadings.


Characterisation
Powder X-ray Diffraction (XRD) XRD data were collected on a PANAnalytical X'Pert Pro diffractometer in reflection mode at 40 kV and 40 mA using Cu Kα radiation (α1 = 1.54060Å, α2 = 1.54426Å, weighted average = 1.54178Å).Scans were recorded from 5° ≤ 2θ ≤ 70°.Samples were mounted on stainless steel sample holders; peaks produced from these holders are observed at approximately 43 and 50°.Inductively Coupled Plasma Mass Spectrometry (ICP-MS) ICP-MS analysis was performed by Dr. Alaa Abdul-Sada at the University of Sussex on an Agilent 7500 Series ICP-MS in helium collision mode.Approximately 30 mg of the sample was dissolved in 10 mL of 10% nitric acid solution.The solutions were then diluted by a factor of 100 with dilute nitric acid prior to analysis.Three repeats were performed for each measurement and the average was recorded.Transmission Electron Microscopy (TEM) All TEM images were obtained on a JEOL 2100 microscope with an accelerating voltage of 200 kV.Samples were prepared by dispersing particles in water or ethanol via sonication for 1 hour before casting onto carbon-coated copper grids.Scanning Electron Microscopy (SEM) All SEM images were obtained on a JEOL JSM 6010LV scanning electron microscope with an accelerating voltage of 15 kV.The powder samples were dispersed in ethanol and then dried on silicon wafer while the epoxy plates were placed directly on the sample holder using carbon tape.All samples were coated with platinum using a Quorum SC 7620 Sputter Coater for 60 s before imaging.
Optical Microscopy All images were obtained on USB Bysameyee Digital Microscope using 8 LED mini video camera.
Optical measurement the optical measurements (total transmittance, diffuse transmittance and absorption) were conducted by using the UV-Vis-NIR spectrophotometer (PerkinElmer Lambda 1050+) with a 150 mm diameter integrating sphere.
Raman Spectroscopy the Raman spectra were collected from epoxy plates using a DXR3 Smart Raman Spectrometer with a 785 nm excitation laser.
Tensile Strength Samples were prepared via high-speed mixing under vacuum and cast in a silicon mold -ISO 37 Type 3 dogbone (width b 1 = 4 mm, length L 0 = 10 mm, thickness ca. 4 mm).Before measurements, samples were conditioned for 7 days at room temperature.The measurements were performed using an Instron 5582 tensile tester equipped with a 5 kN load cell.A grip-to-grip separation of 40 mm was used.The samples were pre-stressed to 3 N, then loaded with a constant cross-head speed of 100 mm/min.To calculate the tensile strength as stress (MPa), the reported force value was divided by the cross-sectional area (ca.20 mm 2 ) of the specimens.The reported values are an average of at least 5 measurements of each composition and reported error ± 1σ.

Laser pulse frequency (kHz) Laser head speed (mm/s)
The calculation is shown here step by step, take a sample square (5000 mm/s and 100 kHz) as example: Power is 20 Watt, Pulse width is 5 μs, spot size of the lens is 40 μm, sample square is 2 mm x 2 mm, the line width is 0.04 mm, frequency is 100 kHz and the speed is 5000 mm s -1 .